For purposes of clarity, the term “conventional engine” as used in the present application refers to an internal combustion engine wherein all four strokes of the well known Otto cycle (the intake, compression, expansion and exhaust strokes) are contained in each piston/cylinder combination of the engine. Each stroke requires one half revolution of the crankshaft (180 degrees crank angle (CA)), and two full revolutions of the crankshaft (720 degrees CA) are required to complete the entire Otto cycle in each cylinder of a conventional engine.
Also, for purposes of clarity, the following definition is offered for the term “split-cycle engine” as may be applied to engines disclosed in the prior art and as referred to in the present application.
A split-cycle engine comprises:
a crankshaft rotatable about a crankshaft axis;
a compression piston slidably received within a compression cylinder and operatively connected to the crankshaft such that the compression piston reciprocates through an intake stroke and a compression stroke during a single rotation of the crankshaft;
an expansion (power) piston slidably received within an expansion cylinder and operatively connected to the crankshaft such that the expansion piston reciprocates through an expansion stroke and an exhaust stroke during a single rotation of the crankshaft; and
a crossover passage interconnecting the compression and expansion cylinders, the crossover passage including a crossover compression (XovrC) valve and a crossover expansion (XovrE) valve defining a pressure chamber therebetween.
U.S. Pat. No. 6,543,225 granted Apr. 8, 2003 to Carmelo J. Scuderi (the Scuderi patent) and U.S. Pat. No. 6,952,923 granted Oct. 11, 2005 to David P. Branyon et al. (the Branyon patent) each contain an extensive discussion of split-cycle and similar type engines. In addition the Scuderi and Branyon patents disclose details of prior versions of engines of which the present invention comprises a further development. Both the Scuderi patent and Branyon patent are incorporated herein by reference in their entirety.
Referring to FIG. 1, a prior art split-cycle engine of the type similar to those described in the Branyon and Scuderi patents is shown generally by numeral 8. The split-cycle engine 8 replaces two adjacent cylinders of a conventional engine with a combination of one compression cylinder 12 and one expansion cylinder 14. A cylinder head 33 is typically disposed over an open end of the expansion and compression cylinders 12, 14 to cover and seal the cylinders.
The four strokes of the Otto cycle are “split” over the two cylinders 12 and 14 such that the compression cylinder 12, together with its associated compression piston 20, perform the intake and compression strokes and the expansion cylinder 14, together with its associated expansion piston 30, perform the expansion and exhaust strokes. The Otto cycle is therefore completed in these two cylinders 12, 14 once per crankshaft 16 revolution (360 degrees CA) about crankshaft axis 17.
During the intake stroke, intake air is drawn into the compression cylinder 12 through an intake port 19 disposed in the cylinder head 33. An inwardly opening (opening inward into the cylinder) poppet intake valve 18 controls fluid communication between the intake port 19 and the compression cylinder 12.
During the compression stroke, the compression piston 20 pressurizes the air charge and drives the air charge into the crossover passage (or port) 22, which is typically disposed in the cylinder head 33. This means that the compression cylinder 12 and compression piston 20 are a source of high pressure gas to the crossover passage 22, which acts as the intake passage for the expansion cylinder 14. In some embodiments two or more crossover passages 22 interconnect the compression cylinder 12 and the expansion cylinder 14.
The volumetric compression ratio of the compression cylinder 12 of split-cycle engine 8 (and for split-cycle engines in general) is herein referred to as the “compression ratio” of the split-cycle engine. The volumetric compression ratio of the expansion cylinder 14 of split-cycle engine 8 (and for split-cycle engines in general) is herein referred to as the “expansion ratio” of the split-cycle engine. The volumetric compression ratio of a cylinder is well known in the art as the ratio of the enclosed (or trapped) volume in the cylinder (including all recesses) when a piston reciprocating therein is at its bottom dead center (BDC) position to the enclosed volume (i.e., clearance volume) in the cylinder when said piston is at its top dead center (TDC) position. Specifically for split-cycle engines as defined herein, the compression ratio of a compression cylinder is determined when the XovrC valve is closed. Also specifically for split-cycle engines as defined herein, the expansion ratio of an expansion cylinder is determined when the XovrE valve is closed.
Due to very high compression ratios (e.g., 40 to 1, 80 to 1, or greater), an outwardly opening (opening outward away from the cylinder) poppet crossover compression (XovrC) valve 24 at the crossover passage inlet 25 is used to control flow from the compression cylinder 12 into the crossover passage 22. Due to very high expansion ratios (e.g., 40 to 1, 80 to 1, or greater), an outwardly opening poppet crossover expansion (XovrE) valve 26 at the outlet 27 of the crossover passage 22 controls flow from the crossover passage 22 into the expansion cylinder 14. As will be discussed in greater detail, the actuation rates and phasing of the XovrC and XovrE valves 24, 26 are timed to maintain pressure in the crossover passage 22 at a high minimum pressure (typically 20 bar absolute or higher during full load operation) during all four strokes of the Otto cycle.
At least one fuel injector 28 injects fuel into the pressurized air at the exit end of the crossover passage 22 in correspondence with the XovrE valve 26 opening, which occurs shortly before expansion piston 30 reaches its top dead center position. The air/fuel charge usually enters the expansion cylinder 14 shortly after expansion piston 30 reaches its top dead center position (TDC), although it may begin entering slightly before TDC under some operating conditions. As piston 30 begins its descent from its top dead center position, and while the XovrE valve 26 is still open, spark plug 32, which includes a spark plug tip 39 that protrudes into cylinder 14, is fired to initiate combustion in the region around the spark plug tip 39. Combustion can be initiated while the expansion piston is between 1 and 30 degrees CA past its top dead center (TDC) position. More preferably, combustion can be initiated while the expansion piston is between 5 and 25 degrees CA past its top dead center (TDC) position. Still more preferably, combustion can be initiated while the expansion piston is between 10 and 25 degrees CA past its top dead center (TDC) position. Most preferably, combustion can be initiated while the expansion piston is between 10 and 20 degrees CA past its top dead center (TDC) position. Additionally, combustion may be initiated through other ignition devices and/or methods, such as with glow plugs, microwave ignition devices or through compression ignition methods.
The XovrE valve 26 is closed after combustion is initiated but before the resulting combustion event can enter the crossover passage 22. The combustion event drives the expansion piston 30 downward in a power stroke.
During the exhaust stroke exhaust gases are pumped out of the expansion cylinder 14 through exhaust port 35 disposed in cylinder head 33. An inwardly opening poppet exhaust valve 34, disposed in the inlet 31 of the exhaust port 35, controls fluid communication between the expansion cylinder 14 and the exhaust port 35. The exhaust valve 34 and the exhaust port 35 are separate from the crossover passage 22. That is, exhaust valve 34 and the exhaust port 35 do not make contact with the crossover passage 22.
With the split-cycle engine concept, the geometric engine parameters (i.e., bore, stroke, connecting rod length, volumetric compression ratio, etc.) of the compression 12 and expansion 14 cylinders are generally independent from one another. For example, the crank throws 36, 38 for the compression cylinder 12 and expansion cylinder 14 respectively may have different radii and may be phased apart from one another such that top dead center (TDC) of the expansion piston 30 occurs prior to TDC of the compression piston 20. This independence enables the split-cycle engine 8 to potentially achieve higher efficiency levels and greater torques than typical four stroke engines.
The geometric independence of engine parameters in the split-cycle engine 8 is also one of the main reasons why pressure can be maintained in the crossover passage 22 as discussed earlier. Specifically, the expansion piston 30 reaches its top dead center position prior to the compression piston reaching its top dead center position by a discreet phase angle (typically between 10 and 30 crank angle degrees). This phase angle, together with proper timing of the XovrC valve 24 and the XovrE valve 26, enables the split-cycle engine 8 to maintain pressure in the crossover passage 22 at a high minimum pressure (typically 20 bar absolute or higher during full load operation) during all four strokes of its pressure/volume cycle. That is, the split-cycle engine 8 is operable to time the XovrC valve 24 and the XovrE valve 26 such that the XovrC and XovrE valves are both open for a substantial period of time (or period of crankshaft rotation) during which the expansion piston 30 descends from its TDC position towards its BDC position and the compression piston 20 simultaneously ascends from its BDC position towards its TDC position. During the period of time (or crankshaft rotation) that the crossover valves 24, 26 are both open, a substantially equal mass of gas is transferred (1) from the compression cylinder 12 into the crossover passage 22 and (2) from the crossover passage 22 to the expansion cylinder 14. Accordingly, during this period, the pressure in the crossover passage is prevented from dropping below a predetermined minimum pressure (typically 20, 30, or 40 bar absolute during full load operation). Moreover, during a substantial portion of the intake and exhaust strokes (typically 90% of the entire intake and exhaust strokes or greater), the XovrC valve 24 and XovrE valve 26 are both closed to maintain the mass of trapped gas in the crossover passage 22 at a substantially constant level. As a result, the pressure in the crossover passage 22 is maintained at a predetermined minimum pressure during all four strokes of the engine's pressure/volume cycle.
For purposes herein, the method of opening the XovrC 24 and XovrE 26 valves while the expansion piston 30 is descending from TDC and the compression piston 20 is ascending toward TDC in order to simultaneously transfer a substantially equal mass of gas into and out of the crossover passage 22 is referred to herein as the Push-Pull method of gas transfer. It is the Push-Pull method that enables the pressure in the crossover passage 22 of the split-cycle engine 8 to be maintained at typically 20 bar or higher during all four strokes of the engine's cycle when the engine is operating at full load.
As discussed earlier, the exhaust valve 34 is disposed in the exhaust port 35 of the cylinder head 33 separate from the crossover passage 22. The structural arrangement of the exhaust valve 34 not being disposed in the crossover passage 22, and therefore the exhaust port 35 not sharing any common portion with the crossover passage 22, is preferred in order to maintain the trapped mass of gas in the crossover passage 22 during the exhaust stroke. Accordingly large cyclic drops in pressure are prevented which may force the pressure in the crossover passage below the predetermined minimum pressure.
The high compression ratio within compression cylinder 12 and the high expansion ratio within expansion cylinder 14 are achieved using, inter alia, a flat-topped compression piston 20 and a flat-topped expansion piston 30, respectively. That is, in prior art split-cycle engines, the tops (or top surfaces) of each of compression piston 20 and expansion piston 30 (i.e., the generally circular sides that face toward the cylinder head 33) are substantially flat surfaces. Cylinder head 33 also typically has a flat bottom surface (i.e., a surface of the cylinder head 33 that faces toward the top surfaces of the compression and expansion pistons) facing toward each of the compression 12 and expansion 14 cylinders, so that the volume in these cylinders is minimized when the pistons 20, 30 are at their respective top dead center (TDC) positions.
XovrE valve 26 opens shortly before the expansion piston 30 reaches its top dead center position. At this time the pressure ratio of the pressure in crossover passage 22 to the pressure in expansion cylinder 14 is high, due to the fact that the minimum pressure in the crossover passage is typically 20 bar absolute or higher and the pressure in the expansion cylinder during the exhaust stroke is typically about one to two bar absolute. In other words, when XovrE valve 26 opens, the pressure in crossover passage 22 is substantially higher than the pressure in expansion cylinder 14 (typically in the order of 20 to 1 or greater). This high pressure ratio causes initial flow of the air and/or fuel charge to flow into expansion cylinder 14 at high speeds. These high flow speeds can reach the speed of sound, which is referred to as sonic flow. This sonic flow is particularly advantageous to split-cycle engine 8 because it causes a rapid combustion event, which enables the split-cycle engine 8 to maintain high combustion pressures even though ignition is initiated while the expansion piston 30 is descending from its top dead center position.
However, high speed (and particularly sonic) flow into expansion cylinder 14 creates a pressure wave, which moves the air/fuel charge across the top surface of expansion piston 30. The pressure wave can cause a peak in pressure and/or temperature at or near the walls of expansion cylinder 14. This peak in pressure and/or temperature can have adverse effects such as causing early detonation of the air/fuel charge prior to spark ignition (i.e., pre-ignition). The risk of pre-ignition can be aggravated if the pressure wave peaks near exhaust valve 34 because exhaust valve 34 has one of the hottest surfaces in expansion cylinder 14. Accordingly, there is a need to guide an air/fuel charge carried by a pressure wave in split-cycle engines such that any peak in pressure and/or temperature does not cause pre-ignition.
Referring to FIG. 2, the position of XovrE valve 26 when the expansion piston 30 of split-cycle engine 8 is approximately at its top dead center position is illustrated. XovrE valve 26 includes a generally disc shaped valve head 40 from which a generally cylindrical valve head stem 41 extends outwardly. When piston 30 reaches its TDC position, the head 40 of XovrE valve 26 is elevated above its closed (or seated) position in cylinder head 33. Curtain areas 42 and 44 are local minimum cross-sectional areas through which fluid can flow. In other words, the curtain areas 42 and 44 are the most potentially restrictive areas to the flow of air/fuel between the crossover passage 22 and the expansion cylinder 14 when the expansion piston 30 is at or near its top dead center position.
The air/fuel charge flowing from crossover passage 22 into expansion cylinder 14 must pass through curtain area 42, which is in the shape of a truncated cone (hereinafter a “truncated conical” shape) between the head 40 of XovrE valve 26 and cylinder head 33. Much of the air/fuel charge flowing from crossover passage 22 into expansion cylinder 14 must also pass through cylindrically shaped curtain area 44 between the expansion piston 30 and the cylinder head 33. The region between truncated conical curtain area 42 and the outlet 27 of the crossover passage 22 is known as the valve pocket 46 of XovrE valve 26. More specifically, the valve pocket 46 is the region bounded by the head 40 of XovrE valve 26, cylinder head 33, truncated conical curtain area 42, and the outlet 27 of the crossover passage 22.
When the expansion piston 30 is at or near its top dead center position the expansion piston clearance 48 (i.e., the clearance depth between the top surface 50 of expansion piston 30 and the bottom surface (or fire deck) 52 of the cylinder head 33, which faces the interior of the expansion cylinder 14) can be very small (e.g., 1.0, 0.9, 0.8, 0.7, or 0.6 millimeters, or less). The distance that XovrE valve 26 opens away from its seated position is known as the valve lift of XovrE valve 26. Notably, the expansion piston clearance 48 can be comparable to, or even less than, the XovrE valve 26 lift. This means that cylindrical curtain area 44 can be comparable in area to, or even smaller than, truncated conical curtain area 42. Such a small cylindrical curtain area 44 can cause a substantial pressure drop and reduction in flow. In other words, when the cylindrical curtain area 44 is comparable in area to truncated conical curtain area 42, the cylindrical curtain area 44 can prevent an appropriate amount of an air/fuel charge from entering the expansion cylinder 14 within appropriate time constraints. This situation is particularly pronounced when the cylindrical curtain area 44 is smaller than the truncated conical curtain area 42 because, in this case, the cylindrical curtain area 44 is the most restrictive area in the flow of air/fuel from the crossover passage 22 into the expansion cylinder 14 when the expansion piston 30 is at or near top dead center.
The above mentioned pressure drop and/or reduction in flow are problematic in that they can reduce engine efficiency. Accordingly, there is a need to increase the size of the curtain area 44 formed between the expansion piston and the cylinder head of a split-cycle engine, so long as the increase in efficiency from doing so is greater than the loss of efficiency caused by the resulting decreased expansion ratio in the expansion cylinder.
XovrE valve 26 must achieve sufficient lift to fully transfer the air/fuel charge in a very short period of crankshaft 16 rotation (generally in a range of about 30 to 60 degrees CA) relative to that of a conventional engine, which normally actuates the valves within 180 to 220 degrees CA. This means that XovrE valve 26 must actuate about four to six times faster than the valves of a conventional engine. Fuel is injected into the exit end of the crossover passage 22 in synchronization with the timing of XovrE valve 26 actuation. Spark plug 32 is fired to initiate combustion shortly thereafter (preferably between 1 to 30 degrees CA after top dead center of the expansion piston 30, more preferably between 5 to 25 degrees CA after top dead center of the expansion piston 30, most preferably between 10 to 20 degrees CA after top dead center of the expansion piston 30).
Given the aforementioned constraints, air/fuel mixing and distribution throughout expansion cylinder 14 must take place in a very short period of time (or crankshaft rotation). Proper distribution of fuel throughout expansion cylinder 14 and optimal air/fuel ratios over the spark-plug(s) 32 should result in improved ignition and more of the available fuel being burned. Accordingly, there is a need to guide fuel distribution in a split-cycle engine to distribute the fuel appropriately throughout the expansion cylinder and improve the air/fuel ratios over the spark plugs.